that engine code (AER) is the 1.0 8v with roughly 50bhp 1996 to roughly 1999
Think they have a distributor in back of cylinder head so possibly get the impulse signal from the Hall sensor,
which in turn tells the ECU and coil pack the engine is rotating,
if the Hall sensor inside distributor fails the engine won't start.
I don't think the (AER) engine has a cam sensor ?
The crank sensor communicates with the ECU and tells the fuel injectors / coil pack when to fire,
to be honest I can't see the crank sensor on ETKA for your engine with code (AER)
I can see the sensors listed for the (ABF) and (AGG) 2.0 engines and (AFT) 1.6 engine,
but not for yours ?
the crank sensor on the larger 2.0 engines is on the front side just behind the oil filter housing,
very close to where the gearbox is bolted up to back face of engine block,
the sensor picks up movement from the toothed wheel on back of crank shaft.
Some of the smaller VW engines have the crank sensor running off the back of flywheel,
which again have a toothed ring the sensor picks up.
If you have a crank sensor on your engine it's possible it might be behind flywheel,
but like I say can't see a crank sensor listed for yours or a cam sensor either on the SEAT ETKA site.
If yours does have a crank sensor it's possibly part number 030 957 147G
It'll be black in colour and possibly fit into back of engine block,
protruding through to pick up off toothed ring on back of flywheel,
or they can be very small and flat and are bolted directly to the back of the engine block rear face,
just above flywheel right at the top, there might be a cut out in top of bell housing where you'll see it,
on some you have to remove the gearbox to gain access to this type of small crank sensor,
which I've seen on the mk3 and mk4 VW Polo's.
Your best bet is to contact your local SEAT Main Dealership to be sure.
On the newer Ibiza's the mk4 / mk5 2008 to 2017 with the 1.0 TSI and 1.2 TSI engines,
they do have a crank sensor which is normally white in colour, and in the shape of the letter Z,
these work of the back of crank shaft / flywheel.